Macrophages occupy a unique niche in the immune system, in that they can not only initiate immune responses but they can also be effector cells which contribute to the resolution of these responses. The immunologic activation of macrophages can cause dramatic differences in their physiology. It is now well-appreciated that there is no single activation phenotype, and that macrophages can be induced to differentiate into cells that can either exacerbate or inhibit inflammation. Similarly, these cells can promote, deviate, or suppress adaptive immune responses. The various states of macrophage activation and the mechanism of their induction will be examined. The influence that each of these altered cell populations exerts on adaptive and innate immune responses will be a particular focus of this meeting. Activated macrophages have long been appreciated to be immune effector cells, capable of exerting potent antimicrobial activities. It is now clear, however, that certain pathogens can actively inhibit specific macrophage functions and exploit these cells for intracellular growth. The mechanisms behind these inhibitory processes will be examined, with a goal to understanding microbial pathogenesis from the perspective of the phagocyte. The overall goal of this meeting is to better understand the complex processes by which the physiology of the macrophage can be so profoundly changed during the process that we call macrophage activation, and how these changes affect immune responses. These discussions should lead to a better understanding of how specific alterations in macrophage physiology can contribute to inflammation, immune deviation, and the elimination of microbial pathogens.